Abstract

Analyses of Co, Ni and Se in pyrites and other minerals from a wide variety of Tasmanian ore deposits support a genetic relationship between the Mt. Lyell (pyritic - Cu) and Rosebery (banded In-Zn-Pb-Cu) deposits, and the Cambrian eugeosynclinal volcanic rocks in which they occur.The concentration trends for all the ores due to fundamental availability contain smaller-scale components due to depositional processes, which, except in one case, do not interfere with these trends. The components include impoverishment of Co (and Ni) during remobilization of sulphides, increase in Ni and decrease in Co away from the centre of zoned deposits, impoverishment of Ni (and Co) in replacement as compared with vein lodes, and regular and irregular partitioning of respectively Co-Ni and Se between coexisting minerals.The trends of Co-Ni in pyrite due to availability include the following:(a) The sedimentary-diagenetic pyrites generally contain Co/Ni < 0.5, but show no correlation of Co-Ni values with rock type, age, or degree of recrystallization.(b) Pyrites in the Cambrian acid-intermediate igneous rocks have Co/Ni ratios ranging 1-150, and contain up to 0.8% Co.(c) Pyrites from Devonian hypogene replacement and vein deposits show two main trends - 0-1500 ppm Ni, with Co/Ni < 1; and 0-400 ppm Co, with Ni ranging 10-100 ppm.The Se contents of sulphides associated with Cambrian and Devonian acid-intermediate igneous activity are indistinguishable, but the Savage River magnetite ore, the Cuni Cu-Ni ore, and particularly the Mt. Lyell ore are all enriched in Se, whereas the Rosebery ore is impoverished. The Co-Ni values in the Mt. Lyell and Rosebery ores follow the trend for pyrites in the Cambrian acid-intermediate igneous rocks. The dispersion of Se at Mt. Lyell is consistent with the postulated open-cast origin for the massive ores, and within the Rosebery lode, the stratification of the Co-Ni values, and their gradation between lode and normal sedimentary types, strongly suggest a sedimentary origin.These and other collated data suggest that ores associated with geosynclinal vulcanism may be characterized by (i) high to very high Co/Ni ratios, and often marked impoverishment in Ni, (ii) greater Co and Ni concentrations associated with Cu than with Pb-Zn ores, both within and between deposits, and (iii) a tendency for Co to correlate with Cu within deposits. In Tasmania, Se values are more closely controlled than Co-Ni in their fundamental availability by other than genetic factors, and within deposits they show less variation due to depositional processes.Selenium was analyzed by X-ray fluorescence spectrography, and Co and Ni by atomic absorption spectrophotometry. In the latter technique, because Fe caused both non-atomic absorption and chemical interferences in the flame, it was extracted with di-isopropyl ether.

Item Type:

Thesis
(PhD)

Keywords:

Ores, Mineralogy

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